CYTODIFFERENTIATION OF THE ODONTOCLAST PRIOR TO THE SHEDDING OF HUMANDECIDUOUS TEETH - AN ULTRASTRUCTURAL AND CYTOCHEMICAL STUDY

Background: In human deciduous teeth, odontoclastic resorption takes p lace at the pulpal surface of the coronal dentine prior to shedding, a nd this resorption shows clear time-related histological changes (Saha ra et al., 1992). Methods: Using this phenomenon as an observation sys tem, we examined the cytodifferentiation of human odontoclasts by ligh t and electron microscopy. For a histochemical marker of odontoclast d ifferentiation and function, tartrate-resistant acid phosphatase (TRAP ) activity was determined by light and electron microscopic enzyme his tochemistry. Results: As root resorption neared completion, TRAP-posit ive mononuclear cells were initially detected in the pulp chamber. The y had abundant mitochondria, small lysosomes, and moderately developed rough endoplasmic reticulum throughout their cytoplasm. In these mono nuclear cells, TRAP activity was localized in compartments of the bios ynthetic pathway, i.e., in cisternae of the endoplasmic reticulum and Golgi lamellae, as well as small lysosomes. The TRAP-positive mononucl ear cells first made contact with the predentine surface by their elon gated cellular processes. After attachment, they spread out along the predentine surface and developed specialized membrane structures, clea r zones, and ruffled borders. Next, they fused with each other on the predentine surface and formed typical multinucleate odontoclasts. Afte r termination of their resorption function, the odontoclasts lost thei r ruffled borders and became detached from the resorbed surface. Most of the detached odontoclasts had numerous large pale vacuoles and seco ndary lysosomes and appeared to be in the process of degeneration. Con clusions: The present study demonstrates that: (1) odontoclasts differ entiated from TRAP-positive mononuclear cells, which presumably origin ate from circulating progenitor cells, (2) membrane specialization of odontoclasts, i.e., development of a clear zone and ruffled border, is induced following their contact with the resorption surface, (3) mult inucleation of odontoclasts takes place only after their attachment to the resorption surface, (4) mature multinucleate odontoclasts can res orb predentine as well as dentine in the same way as osteoclasts resor b bone, and (5) at the end of the resorption, odontoclasts gradually l ose their ruffled borders and become detached from the resorbed surfac e. (C) 1996 Wiley-Liss, Inc.